The subendocardium is more vulnerable to ischemia than is the epicardium. Transmural gradients in intramyocardial pressure, oxygen tension, myocardial blood flow and glycolytic enzyme activities have been documented. Although ischemia causes acidosis, the existence of a transmural gradient of pH has not been documented or quantitated. By means of specially designed fiberoptic pH probes implanted in the ischemic and normal canine myocardium, we documented the existence of a large transmural pH gradient over 3-4 mm. The close spatial proximity of regions with markedly different pH undoubtedly contributes to physiological abnormalities and arrhythmogenesis in ischemic hearts.